Local Anaesthetics Flashcards
What is local anaesthesia? 2
- Prevents transmission of the nerve impulse in the region it is applied.
- Reversible loss of sensory information where the neuron is not damaged
What is the chemical structure of local anesthetics?
- Weak bases
- Lipophilic ring
- Hydrophilic amine
- Intermediate chain is either an amide or ester
What are the two classes of local anaesthetics?
- Esters
2. Amides
What are the 3 types of ester LA?
- Long action (letracaine)
- Short action (procaine)
- Surface action (benzocaine, cocaine)
What are the two types of amides?
- Long action (bupivacaine, ropivacaine)
2. Medium action (lignocaine)
What is the MOA of local anaesthetics? 2
- Blocks voltage gated sodium channels which reduces the influx of sodium ions
- This prevents the depolarization of the membrane blocking conduction of the action potential
Describe the importance of the pKA of LA 4
- All LA are weak bases
- Exist in 2 forms: unionized (B) and ionized (BH+)
- The pKA of a weak base defines the pH at which both forms exist in equal amounts
- As the pH of tissues differs from the pKa of specific drug, more of the drug exists in either its charged or uncharged forms
Explain the effect of pH on LA using infected tissue as an example 3
- All LA are weak bases and therefore, are unionized in basic conditions
- When tissue is infected, the pH will be lower (more acidic) which means more of the LA will be ionized and have less of an effect
- Lastly, there will be an increased blood supply to the affected tissue and thus, LA will be more readily removed from the target area
What effects the potency of LA?1
The more lipophillic the drug is the more easily it can pass through the cell membrane
What affects the duration of action?
The higher the protein binding, the longer the duration of action
Explain the use of a vasoconstriction in conjunction with LA 4
The addition of a vasoconstrictor will:
- Localize the anaesthetic at the desired site
- Prolong the duration of action
- Reduce local haemorrhage
- Decrease toxicity
What precautions must one be aware of before using a vasoconstrictor? 4
- Tricyclic antidepressants
- Non-selective beta blockers
- Cocaine/ methamphetamine abusers
- Digoxin
What are the contraindications to vasoconstrictor use? 5
- Severe heart disease
- Uncontrolled hyperthyroidism
- Sulphite allergies
- Asthmatics
- Pheochromacytoma
How are amides metabolized?
In the liver
How are esters metabolized
Pseudocholinesterase
Which nerve fibres are most readily blocked by LA?
Small diameter nerve fibers such as c-fibers are more readily blocked than large fibers
Generally, what is the order in which fibres are blocked? 5
- Sympathetic and pain
- Temperature
- Light touch and proprioception
- Motor
Recovery occurs in the reverse order
Name a topical anaesthetics? 1
- Lignocaine (amide)
Name the injectable LA 3
- Lignocaine (amide)
- Bupivacaine (amide)
- Ropivacaine (amide)
Describe the administration of topical local anaesthesia 3
- High conc of oily vehicle slowly penetrates the skin or mucous membrane
- Provides short term pain relief
- Can be applied prior to a minor procedure or venepuncture
Describe the administration of infiltration local anaesthesia 3
- Local anaesthesia is injected intradermally or subcutaneously by the area to be anaethetized
- Produces numbness faster than topical anaesthesia
- Used extensively in dentistry
Describe the administration of peripheral nerve block local anaesthesia 2
- Injection of LA solution around a nerve trunk
2. Produces a field of anaesthesia distal to site of injection
Describe the administration of central nerve block local anaesthesia 3
- LA is injected near the spinal cord
- Produces anaesthesia above and below the site of injection
- Epidural anaesthesia used extensively during labour
Describe the administration of intravenous regional local anaesthesia 2
- LA is injected into a limb after application of a tourniquet
- Used for manipulations of fractures and minor surgical procedures
What types of adverse effects are associated with LA? 3
- Toxicity: cns and cvs
- Allergy
- Methhaemoglobinaemia
Describe CNS effects of LA toxicity 5
- Tingling lips
- Slurred speech
- Reduced levels of consciousness (treat with oxygen)
- Seizures (treat with benzodiazepines)
- Resp failure
Describe the CVS effects associated with LA toxicity 4
- Reduced myocardial contractility
- Arrhythmias
- Hypotension (treat with vasopressors)
- Bupivacaine induced ventricular fibrillation (treat with an intralipid infusion)
What are the uses of lignocaine?2
- LA
2. Anti-arrhythmic
How is lignocaine metabolized
Lignocaine is dealkylated in the liver by CYP. The metabolites retain LA activity
What does lidocaine toxicity cause? 4
- Dizziness
- Drowsiness
- Tinnitus
- Twitching
What are the contraindications for lignocaine?2
- Heart block
2. Conduction disturbances
Describe the PK of bupivacaine 3
- Widely used during labour or post-operatively
- Long lasting (high protein binding)
- Slow onset
What are the indications for bupivacaine?4
- Eye block
- Spinal block
- Epidural
- Caudal block
How id bupivacaine metabolized
Liver cyp
How is bupivacaine excreted?
Urine
